1. Field of the Invention
The present invention relates to a photovoltaic element capable of improving the photoelectric converting efficiency by a light enclosing effect, while improving also the yield in the manufacture, the weather resistance and the durability.
2. Related Background Art
There is already known a technology of forming, on a side of the photovoltaic element opposite to the light entrance side, a reflective layer composed of a metal film of high reflectance such as Ag, Al, Cu or Au. This technology serves to reflect the light, transmitted by the carrier-generating semiconductor layer, to stimulate the absorption again in the semiconductor layer, thereby increasing the light absorption in the thin semiconductor layer, thus increasing the output current and improving the photoelectric converting efficiency.
Also a transparent conductive layer may be provided between the rear electrode and the semiconductor layer to provide the following effects:
(1) improving flatness of the rear electrode, improving adhesion of the semiconductor layer and/or preventing alloy formation between the metal of the rear electrode and that of the semiconductor layer (cf. Japanese Patent Publication Nos. 59-43101, (Fuji Electric Mfg. Co. and 60-41878 Sharp Corp.); PA1 (2) decreasing the current in the defect area of the semiconductor layer (Japanese Patent Application Laid-open No. 60-84888, Energy Conversion Devices, Inc.); and PA1 (3) increasing the sensitivity in the long-wavelength region within the spectral sensitivity range of the solar cell (Y. Hamanaka et al., Appl. Phys. Lett., 43(1983) p. 644).
It is also known to form, on the rear electrode, a light-scattering texture in the order of wavelength of the scattered light, thereby scattering the long-wavelength light that cannot be absorbed in the semiconductor layer and extending the optical path length in the semiconductor layer, thereby improving the long-wavelength sensitivity of the photovoltaic element, increasing the short-circuit current and improving the photoelectric converting efficiency (T. Tiedje et al., Proc. 16th IEEE Photovoltaic Specialist Conf. (1982) p. 1423, H. Deckman et al., Proc. 16th IEEE Photovoltaic Specialist Conf. (1982) p. 1425).
By combining these technologies, an optimum configuration is considered to be provided with a metal film, serving as the rear reflective layer and also as the rear electrode, having a light-scattering surface texture in the order of the wavelength of the light and also having a high reflectivity, and also with a transparent conductive layer positioned between the rear reflective layer and the semiconductor layer.
FIG. 2A is a cross-sectional view of a conventional photovoltaic element, wherein provided in succession, on a substrate 201, are a transparent conductive layer 202, a semiconductor pin junction 203-205, a transparent electrode 206 and a current-collecting electrode 207. In another configuration shown in FIG. 2B, a metal layer 208 is provided between the substrate 201 and the transparent conductive layer 202.
However the manufacture of the photovoltaic element with the transparent conductive layer and the metal layer of the prior art has resulted in certain drawbacks in terms of process efficiency and durability.
In case of forming the semiconductor layer on a surface having pyramidal texture (cf. Tiedje et al., Proc. 16th IEEE Photovoltaic Specialist Conf. (1982) p. 423), a stress is generated at the pyramidal vertexes, inducing the formation of a defect in the semiconductor layer. Also upon voltage generation, the electric field is concentrated at the pyramidal vertexes to increase the leak current of the photovoltaic element through the defect or the like of the semiconductor layer, whereby the production yield of the photovoltaic element is deteriorated.
Also the semiconductor layer formed on the surface having pyramidal texture shows a stronger electric field at the pyramidal vertexes in comparison with the semiconductor layer formed on a flat surface, and the resulting uneven electric field tends to deteriorate the open-circuit voltage (Voc) and the fill factor (FF) of the photovoltaic element, in comparison with that formed on the surface of a flat substrate.
Also the photovoltaic element may show increases in the photodeterioration (deterioration of the element characteristics by prolonged light irradiation) and deterioration by vibration (deterioration of the element characteristics by prolonged vibration). The photodeterioration of the photovoltaic element is assumed to result from cleavage of weak bonds by the light energy, constituting recombination centers for the photoinduced carries and thus deteriorating the characteristics of the element. Also the deterioration of the photovoltaic element by vibration is assumed to result from cleavage of weak bonds by the vibrational energy, constituting recombination centers for the photoinduced carries and thus deteriorating the characteristics of the element. Such weak bonds are considered to be localized in the area where a stress is generated.
Also when Ag or Cu is employed in the rear reflective metal layer, it has been found, if the humidity is high and a positive bias voltage is applied to the rear reflective metal layer, that Ag or Cu migrates to form conductive tracts to the electrode at the light entrance side, inducing shunt or short-circuiting of the photovoltaic element. Such a phenomenon is particularly conspicuous in case the rear reflective metal layer has a surface texture in the order of the wavelength of the light.
On the other hand, the rear electrode formed flat is associated with drawbacks of insufficient light absorption in the semiconductor layer because of the limited light scattering at the rear side and of insufficient adhesion between the substrate and the rear reflective layer, depending on the combination of the materials constituting the substrate and the rear electrode, eventually leading to the peeling of the rear reflective layer from the substrate in the post-working steps of the photovoltaic element.
Furthermore, in a post-working step for eliminating the electrical shunt in the defect area, in case of a surface with pyramidal vertexes, the reaction may proceed excessively at the vertex of the isolated steep pyramid, thus causing damage to the defect-free area. In such substrate, this fact limits the range of condition setting in the above-mentioned step of eliminating the electrical shunt in the defect area. Consequently this fact necessitates stricter control of the production process, thus deteriorating the productivity.
The above-mentioned drawbacks become particularly conspicuous when the production process is designed for lost cost for realizing practical use of the photovoltaic elements, such as the use of an inexpensive substrate such as a resinous film or a stainless steel plate, or of a higher formation speed of the semiconductor layer, and are factors deteriorating the production yield of the photovoltaic elements.